1. Field of the Invention
The present invention is directed to stable, colloidal, aqueous dispersions of cross-linked urea-urethane polymers and their method of production.
2. Description of the Prior Art
Urea-urethanes are used for a wide range of commercial applications such as adhesives or coatings for various substrates including textile fabrics, plastic, wood, glass fibers and metals. Chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability are among the many desirable properties of these coatings.
Conventionally, coatings of urea-urethanes have been formed from solutions of the urea-urethanes in organic solvents. As the coating cures, the solvents evaporate into the atmosphere. This is economically disadvantageous due to the high costs of these solvents, but more importantly, these solvents also cause pollution of the atmosphere.
Accordingly, many efforts have been directed to forming urea-urethane coatings from dispersions of these polymers in water. Economically, the use of water is very advantageous and, in addition, water does not pollute the atmosphere as it evaporates from the coating. However, urea-urethanes are not compatible with water, i.e., they do not form stable dispersions in water, unless special processes and/or specific ingredients are used in their manufacture.
Early methods of forming stable dispersions of urea-urethanes used external emulsifiers to disperse and stabilize these polymers in water. Note U.S. Pat. No. 2,968,575 to Mallonee. While the use of these emulsifiers made it possible to produce stable dispersions, coatings produced therefrom had many disadvantages, including sensitivity to water, due to the presence of these emulsifiers.
Continued efforts led to the use of emulsifiers which were chemically incorporated into the backbone of the urea-urethane polymers. Incorporated ionic emulsifiers were disclosed in U.S. Pat. No. 3,479,310 to Dieterich et al. The use of nonionic emulsifiers chemically attached to the backbone of the polymer, is disclosed in U.S. Pat. Nos. 3,905,929, 3,920,598 and 4,190,566 to Noll, Reiff et al and Noll et al, respectively. Further improvements in the properties of coatings prepared from urea-urethane polymers were obtained by combining both ionic groups and nonionic groups into the polymer backbone. Note U.S. Pat. Nos. 4,092,286 and 4,237,264 to Noll et al and U.S. Pat. No. 4,238,378 to Markusch et al.
U.S. Pat. No. 4,066,591 to Scriven et al is directed to the production of urea-urethanes by forming isocyanate-terminated prepolymers, dispersing them in water and subsequently chain extending the prepolymers with diamines. The final product is an aqueous dispersion of a substantially linear polymer due to the fact that the functionality of the prepolymer components and the chain extender is maintained at essentially two to avoid the production of gelled or cross-linked particles in the dispersion. The products are soluble in organic solvents such as N-methyl-2-pyrrolidinone or dimethyl formamide.
The previously mentioned patents are directed to the formation of linear polyurethanes or urea-urethanes. While these products are sufficient for a number of uses, coatings derived from linear polyurethanes do not possess good resistance to organic solvents. U.S. Pat. No. 4,203,883 to Hangauer, Jr. discloses that the solvent resistance, among other properties, may be improved by using polyfunctional amines as chain extenders in order to produce cross-linked urea-urethanes. U.S. Pat. No. 3,870,684 to Witt et al and equivalent British Pat. No. 1,143,309 also disclose the use of polyfunctional amines for producing cross-linked urea-urethanes. However, both Hangauer, Jr. and Witt et al exclusively use ionic groups to ensure to the dispersibility of the urea-urethane in water. As a result, the aqueous polyurethanes obtainable are unstable to changes in pH or towards the addition of salt group-containing additives which are useful in coatings or adhesive formulations. Examples include the addition of alkoxysilanes requiring specific pH values, the addition of melamine-formaldehyde adducts together with organic sulfonic acids or sulfonic acid salts, and the addition of polyvalent cations to provide cross-linking. Hangauer, Jr. also recommends against the use of excessive amounts of ether groups since they cause the resulting polymers to swell in water.
It is an object of the present invention to provide aqueous dispersion of urea-urethanes which possess an improved ability to coalesce to films or coatings.
It is a further object of the present invention to provide aqueous dispersions which are stable over a wide range of temperature and pH conditions.
It is an additional object of the present invention to provide aqueous dispersions of urea-urethanes which remain stable in the presence of electrolytes.
It is also an object of the present invention to react isocyanate-terminated prepolymers with polyfunctional amine chain extenders in a manner which produces stable, colloidal aqueous dispersions.
It is a final object of the present invention to use aqueous dispersions of urea-urethanes to produce coatings or films which possess a wide range of properties including hydrolysis resistance, solvent resistance, hardness, elasticity, flexibility, tear resistance, tensile strength and resistance to yellowing.
These objects may be achieved according to the present invention by reacting aqueous dispersions of substantially linear, isocyanate-terminated prepolymers containing nonionic and optionally, ionic, chemically incorporated, hydrophilic emulsifiers with polyfunctional amine chain extenders to produce cross-linked urea-urethane particles which are colloidal in size and remain stably dispersed in water.